This application claims the priority of DE 196 11 360.1, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a device for automatic actuation of a hydraulic brake system of a road vehicle for the purpose of maintaining a vehicle stationary for the duration of traffic-related stationary phases by coupling brake pressure from a brake-pressure source which can be activated without the cooperation of the driver into at least one, and preferably all, of the wheel brakes of the vehicle.
DE-43 32 459 A1 describes an actuating device for a road vehicle with an automatic transmission. The known actuating device has a brake-pressure control device. By processing sensor output signals of a sensor arrangement in accordance with plausibility criteria while the engine is running and with the vehicle in a stationary condition achieved after a braking operation, automatic continuation of the stationary braking operation is provided. After a delay time period, .delta.t, of defined length since the time the vehicle came to a halt has passed, a brake pressure sufficient to keep the vehicle reliably in the stationary condition is coupled into the wheel brake(s) used for the stationary braking operation.
Here, a signal which triggers the stationary braking mode is obtained from a conjunctive (AND logic) linking of the following signals:
(a) An output signal of an electronic speed or wheel-speed sensor, characteristic of the stationary condition of the vehicle, PA1 (b) an output signal of an electronic engine speed transmitter and/or a pressure sensor which monitors the pressure at the inlet branch of the vehicle, as an information signal for a running engine, PA1 (c) an output signal characteristic of the actuated state of the brake pedal, and the output signal of the brake light switch and/or that of an electronic pressure transducer by way of which the pressure in at least one of the wheel brakes can be detected, and PA1 (d) an output signal of a transmission-ratio transmitter.
The release of the brakes required upon resumption of the driving mode is controlled by the control device, which, from a redundant combination of sensor output signals, e.g. those of an accelerator-pedal position transmitter and a throttle-valve position sensor or an engine-speed sensor, detects the driver's desire for resumption of the driving mode.
The known actuating device is designed so that the driver is initially merely assisted in actuating the brakes. After the expiration of a delay time period which is in practice short, i.e. 1 to 2 seconds, however, the brake pressure is increased to such an extent that the vehicle is held stationary in a reliable manner, even on a hill, and the driver can take his or her foot from the brake pedal. The brake is released again only when the driver actuates the accelerator pedal and the throttle-valve position sensor produces a signal characteristic of this.
In stationary phases at traffic-light-controlled crossroads or in slow-moving motorway traffic involving numerous stationary phases, the known actuating device provides the desired relief for the driver. However, the automatic activation of a stationary braking operation with a relatively high braking force is problematic in parking situations, especially on an inclined road surface. This occurs for example, in reverse parking, the driver, after having placed his or her vehicle next to the vehicle in front, would like to allow the vehicle to roll backwards with a forward drive ratio engaged in order to avoid the engagement of the reverse drive ratio but there is an associated risk that the vehicle will start to roll with a jerk as soon as the accelerator pedal is actuated. On a flat road surface, too, parking of the vehicle when a number of changes between forward and reverse travel are required can be made more difficult by the actuating device.
It is, therefore, an object of the present invention to improve an actuating device in such a way that, without prejudice to required triggering of stationary braking-mode phases, parking operations can be carried out in a simple and reliable manner.
This object has been achieved according to the present invention by providing that the brake-pressure control device is configured to trigger the automatic stationary braking mode only if, within the delay time period, the driver performs an actuating action which can be recognized as a driver's desire for triggering of automatic stationary braking and is detectable by the sensor arrangement, and provides the ending of the automatic stationary braking mode only if, in addition to the sensor output signals of the sensor arrangement which reveal the driver's desire for resumption of the driving mode, there are also present sensor output signals which, again with reference to or initiation actuation criteria, fulfill safety-relevant secondary conditions.
According to the present invention, the triggering of the otherwise automatic stationary braking mode takes place only when the driver himself/herself performs an actuating action within the delay time period, .delta.t, preferably increasing the brake pressure by a minimum amount of about 5 bar by actuation of the brake pedal. The additional effort required for actuation in this situation is marginal because the driver is, in any event, still actuating the brake pedal. It is perfectly reasonable to expect this of the driver in view of the advantage that an automatic stationary braking operation is only carried out when the driver expressly so intends. This also applies if the driver additionally or alternatively actuates a switch designed, for example, as a momentary-contact switch and hence gives an electronic control unit a command pulse which clearly expresses the driver's intention, an intention which can, as it were, still be "expressed", i.e. made effective, after expiration of the delay time period.
Whenever the automatic stationary braking mode is activated, it is advantageous if the occurrence of a speed signal of a velocity (or v) sensor triggers an automatic increase of the braking force until the stationary condition of the vehicle has been achieved again. The brake-pressure increase required for this takes place in steps of about 5 bar in each case. In a preferred configuration of the brake-pressure control device, an additional brake-pressure raising step takes place after the stationary condition of the vehicle has been achieved again. The automatic stationary braking mode ends only if, in addition to sensor output signals of the sensor arrangement which reveal the driver's desire for resumption of the driving mode, there are also present sensor signals which, again with reference to plausibility criteria, signal the fulfillment of safety-relevant secondary conditions, in particular that the bonnet or hood is closed, the driver's seat is occupied and the doors of the vehicle are closed. If one of these conditions is not met, the driver is warned by an acoustic or optical signal, and the driving mode cannot be effected.
Components of the service brake are used to maintain a stationary condition of the vehicle achieved by a targeted braking operation, e.g. by driving inlet valves of an anti-lock system present on the vehicle into their brake-pressure-holding position or by switching solenoid valves. Thereby, the brake circuits of the vehicle are connected to the associated pressure outputs of the brake master cylinder, into their shut-off position, with an advantageously low outlay on electronic and/or electrohydraulic circuitry.
A configuration of the actuating device which is functionally reliable and simple in terms of circuitry is also possible by providing a hydraulic auxiliary pressure source which can be activated by driving the actuating device with an output signal of the electronic control unit and the output pressure of which can be coupled into at least one of the brake circuits for the duration of the automatic stationary braking mode.
In a preferred configuration of the actuating device, the service brake system of the vehicle is activated in the automatically controlled stationary braking mode by driving its pneumatic brake booster.
In the automatic stationary braking mode of the brake system, it is particularly advantageous if the brake pressure used for this purpose is limited to ensure that this pressure can be dissipated again with sufficient rapidity upon the transition to the driving mode.
As an alternative, or in addition to the possibilities mentioned for producing braking force in the stationary braking mode, it is also within the contemplation of the present invention to provide an electric-motor-type or electrohydraulic or electropneumatic actuating element which can be driven by an output signal of the electronic control unit and by way of the driving of which the parking brake of the vehicle can be activated.
In a preferred configuration of the actuating device, an introductory brake-pressure build-up for the stationary braking mode can be followed by the ending of this pressure build-up because the vehicle can be held stationary by the "locked-in" brake pressure maintained in the wheel brakes. It can be expedient here if pressure-build-up cycles with following holding cycles are repeated from time to time, e.g. at one-minute intervals.
If the brake-pressure dissipation which concludes the automatic stationary braking mode is effected by multiple pulsed switching of the change-over valve assigned to the brake circuit used for the stationary braking operation, potential pressure surges and the associated generation of noise can be moderated. This occurs upon the transition from the stationary braking mode to the driving mode, where the electronic control unit first produces a control signal which effects an increase of the output pressure of the brake unit and only then produces the sequence of pulses which serves to drive the respective change-over valve and produces the brake-pressure reduction.
A brake-pressure dissipation which is rapid but nevertheless associated with comparatively little noise generation when driving away again is also achieved, in a vehicle which is equipped with an anti-lock system which operates on the return-pumping principle. This is achieved by the return pump(s) of the system and its brake-pressure regulating valves being driven into the pressure reduction mode for a minimum time period.
The high transmission or drive-ratio reduction makes it advantageous, particularly when ending the automatic stationary braking mode by a transition into the reversing mode, for the brake-pressure dissipation to take place in steps of about 5 bar in order to avoid jerky driving away of a vehicle.